Expandable styrene polymer particles are conventionally prepared by means of a process of dispersing styrene polymer particles in water to from a suspension, wherein an expanding agent is then added to impregnate the particles as disclosed in the specification of U.S. Pat. Nos. 2,893,963 and 2,950,261. But there has been a tendency of such particles, during the impregnation, to agglomerate or stick together to form large clumps, because of the solvent action of the expanding agent during the impregnation of the polymer particles with the expanding agent.
In order to prevent agglomeration of the polymer particles during the step of impregnation, suspending agents are generally used. As the suspending agents, the following compounds are well known, for example, partially saponified polyvinylalcohol, polyacrylate, polymethacrylate, polyvinylpyrrolidone, carboxymethyl cellulose, ethyl cellulose, methyl cellulose, sodium alginate which are water-soluble organic compounds having protective colloidal properties. The above compounds are generally used together with surface active agents or sodium pyrophosphate etc. in order to increase their suspension activity.
There is a tendency, however, to produce agglomerated particles when using the above-described water-soluble organic compounds as suspending agents at a higher temperature, and it is necessary to carefully add the expanding agent in parts or a little at a time continuously when using an expanding agent which is in liquid form at ordinary conditions, e.g. room temperature and atmospheric pressure, such as n-pentane. Such restrictions in the step of impregnation are disadvantageous, since they cause lower producitivity. Furthermore, this presents environmental problems, since it is very difficult to remove the waste water after impregnation.
Suspending agents are disclosed in the specifications of U.S. Pat. No. 2,983,692 and Great Britain Pat. No. 1,202,833: e.g. slightly water-soluble, fine powders of inorganic substances such as tertiary calcium phosphate, magnesium pyrophosphate, calcium carbonate, calcium silicate and bentonite, metal salts of an aliphatic acid such as calcium stearate and zinc stearate, and organic fine powders such as the bis-amide of an aliphatic acid, for example, ethylene-bis-stearoamide, etc. However, the above-mentioned suspending agents are not economical, since each of the above-mentioned suspending agents must be used in large quantities, i.e. no less than 1% by weight of styrene polymer particles, when used alone, in order to prevent the agglomeration of the polymer particles caused by the expanding agent and/or softening agent.
Therefore, a method has been generally adopted of using the fine powders in combination with a surface active agent in order to increase the suspension activity. For example, U.S. Pat. No. 2,687,408, U.S. Pat. No. 2,715,118 and U.S. Pat. No. 2,983,692 show that good results were obtained by using anionic surface active agents with inorganic fine powders. Especially stable suspension activity was obtained by using smaller amounts of a suspending agent consisting of phosphate or pyrophosphate, such as tertiary calcium phosphate, hydroxyapatite and magnesium pyrophosphate in combination with a small quantity of sodium dodecylbenzenesulfonate or sodium laurylalcohol-sulfate ester. When using organic fine powders such as calcium stearate and ethylene-bis-amide, not only may anionic surface active agents be used but also nonionic and amphoteric surface active agents may be used in order to increase the suspension activity. However, such suspending agents are generally not excellent agents.
Surface active agents have been used in combination with slightly water-soluble, fine powders as suspending agents in order to increase the suspension activity. However, it is very difficult to recover and/or release the expanding agent after the impregnation step, due to the vigorous foaming caused by the remaining expanding agent when using a surface active agent and an expanding agent such as propane or butane. It is necessary, therefore, to use an anti-foam agent in order to reduce the amount of foam. It is also difficult to completely remove the waste water containing the surface active agent by ordinary equipment used for treating waste water.
Inorganic compounds including phosphor, such as phosphate or pyrophosphate, become one of the nutritive substances when discharged into a river or lake and such discharging will by limited by means of the present invention to protect the environment in the future.
After investigating a process for improving the above processes, the inventors of the present invention have found that excellent suspension stability is obtained by using a small amount of calcium hydroxide as the suspending agent, without the need of using it in combination with surface active agents.